a Billing Account is a record type added when using SuiteBilling > https://netsuite.custhelp.com/app/answers/detail/a_id/85977/kw/billing%20account it mainly supports the subscription features but also allows for more than 1 'billing account' per customer record.

Ah. Haven't worked with SuiteBilling yet - I was thinking it was something else, glad I asked for clarification!